It is well known that backing up a vehicle with a trailer is difficult and frustrating. Much of the difficulty associated with backing up a trailer results from the fact that it is not intuitive for drivers to sense the jackknifing situation before it is too late and from the fact that many drivers do not know how to steer properly in order to align the trailer to avoid a jackknifing situation. In general, vehicle-trailer backing up is by nature an unstable motion. Jackknifing occurs when a vehicle-trailer is approaching away from its equilibrium position, a position intended by the driver through his or her steering input, and thus becomes unstable. The relative angle between vehicle and trailer diverges from the driver's intended target angle, and usually increases if proper steering and/or braking action are not taken. This is typically out of control by the driver, either due to lack of sufficient driving skill, or the condition is too severe.
U.S. Pat. No. 5,558,351 describes an anti jackknife assembly for a vehicle-trailer, but it is operable only when the trailer is being towed in the forward direction. The trailer hitch has an arcuate extension which extends away from the ball joint. The tongue of the trailer hitch has a ball socket which engages the ball joint, and also has a solenoid device which engages the tip of the arcuate extension when energized and limits the degree to which the ball socket can pivot on the ball joint.
The use of caster type wheels on trailers are known to be useful to prevent jackknifing during backing-up the trailer. U.S. Pat. No. 2,475,174 discloses a trailer having a pair of caster-type wheels, with the trailer having a single central hitch for attachment to the towing vehicle. The trailer also has a pair of separate auxiliary connectors disposed on opposite sides of the hitch adjacent the ends of the vehicle bumper. The auxiliary connectors control a flexible cable linkage, which in turn controls the swiveling of the casters. A three-point hitch arrangement is disclosed, and does not permit free swiveling of the casters, but rather provides a complex arrangement for controlling swiveling of the casters. U.S. Pat. No. 3,033,593 discloses a trailer having a pair of caster-type wheels, a central hitch of conventional configuration, and a pair of sidewardly spaced tie bars so as to create a three-point connection. This arrangement rigidifies the trailer relative to the vehicle, but the side tie bars must be released when swiveling of the wheels and backing of the trailer is desired. Further, this three-point connection does not permit proper rolling movement of the trailer relative to the vehicle, such as may be necessary to compensate for irregularities in the highway. U.S. Pat. No. 4,305,602 discloses a trailer having caster-type wheels, with the trailer having a dual hitch arrangement so as to rigidify the trailer relative to the vehicle. This double hitch arrangement, however, has several disadvantages. The tow vehicle must be provided with a specialized tow bar which mounts hitch balls thereon. Further, the overall hitch arrangement does not permit or compensate for rolling movement of the trailer relative to the vehicle due to unevenness in the highway. This hitch arrangement also does not permit even weight distribution on the trailer or vehicle wheels. U.S. Pat. No. 4,512,593 discloses two caster wheels joined together by drive sprockets and chains so that the caster wheels are constrained for simultaneous movement. The trailer also has two rigid tow bars which couple to sidewardly-spaced hitch balls mounted on the tow vehicle. This arrangement does not permit relative roll between the vehicle and trailer.
What is needed is a simple, convenient means of preventing jackknifing automatically during backing up, but which allows the trailer to pivot freely on a single ball joint when moving in the forward direction.